Ultrasonic flow meters are well known and generally fall into two categories, namely transit time ultrasonic flow meters and Doppler ultrasonic flow meters.
FIG. 1 illustrates the operation of a typical transit time ultrasonic flow meter for measuring the flow of a fluid through a pipe 10. A direction of flow of the fluid is shown by arrow X. The ultrasonic flow meter employs a pair of ultrasonic transducers 12 and 14 attached to an external surface 16 of the pipe 10. The transducers 12 and 14 are placed on opposite sides of the pipe 10 a distance L apart, with the first transducer 12 in an upstream position and the second transducer 14 in a downstream position.
To make a downstream flow measurement, a downstream ultrasonic signal Y is emitted from the upstream transducer 12 towards the downstream transducer 14 at an angle θ to the direction of flow X. A transit time td for the downstream signal Y to reach the downstream transducer 14 is measured.
Similarly, to make an upstream flow measurement, an upstream ultrasonic signal Z is emitted from the downstream transducer 14 towards the upstream transducer 12 at an angle θ to the direction of flow X. A transit time tu for the upstream signal Z to reach the upstream transducer 12 is measured.
The transit times tu and td are given by:
                              t          u                =                  L                      c            +                          v              ⁢                                                          ⁢              cos              ⁢                                                          ⁢              θ                                                          (        1        )                                          t          d                =                  L                      c            -                          v              ⁢                                                          ⁢              cos              ⁢                                                          ⁢              θ                                                          (        2        )            where c is the speed of sound in the fluid and v is the flow speed. A transit time difference Δt is then given by:
                              Δ          ⁢                                          ⁢          t                =                                            t              u                        -                          t              d                                =                                    2              ⁢              v              ⁢                                                          ⁢              L              ⁢                                                          ⁢              cos              ⁢                                                          ⁢              θ                                                      c                2                            -                                                v                  2                                ⁢                                  cos                  2                                ⁢                θ                                                                        (        3        )            Usually the speed of sound is much larger than the flow speed (i.e. v<<c). For example, pure water at 20° C. has a sound speed of 1482 ms−1 as compared to a typical flow speed of 0.01 to 4 ms−1. Therefore, Equation (4) approximates to:
                              Δ          ⁢                                          ⁢          t                ≈                              2            ⁢            vL            ⁢                                                  ⁢            cos            ⁢                                                  ⁢            θ                                c            2                                              (        4        )                                ⇒                  v          ≈                                                    c                2                            ⁢              Δ              ⁢                                                          ⁢              t                                      2              ⁢              L              ⁢                                                          ⁢              cos              ⁢                                                          ⁢              θ                                                          (        5        )            Furthermore, the speed of sound may be calculated from an average of the upstream and downstream transit times tu and td:
                              t          av                =                                                            t                u                            +                              t                d                                      2                    =                                    Lc                                                c                  2                                -                                                      v                    2                                    ⁢                                      cos                    2                                    ⁢                  θ                                                      ≈                          L              c                                                          (        6        )                                ⇒                  c          ≈                      L                          t              av                                                          (        7        )            Thus, substituting Equation (7) into Equation (5), the flow speed may be calculated from the measured transit times as follows:
                    v        ≈                              L            ⁢                                                  ⁢            Δ            ⁢                                                  ⁢            t                                2            ⁢                          t              av              2                        ⁢            cos            ⁢                                                  ⁢            θ                                              (        8        )            
Ultrasonic flow meters can be implemented in many ways. Most commonly they are used in industrial applications where alternative measurement technologies cannot be used or where high performance is required. However, such industrial ultrasonic flow meters are not suitable for high volume production or consumer markets due to their physical constraints, power consumption or cost of manufacture.
The object of this invention is to overcome the constraints of industrial ultrasonic flow meters and to provide a method and system of ultrasonic flow measurement that can operate at low power. In particular, it is desired to provide a system which can operate from a battery for up to 10 years and can be manufactured at low cost.